Technical Description

The 24 hour measuring is comprised of:

Diagram 1 - Monitoring station

(a) Remote monitoring stations (Diagram 1) which are installed at various locations throughout the country (primarily schools) and record electromagnetic radiation levels in the environment.

(b) Two central control stations (PCs) installed in the two university labs which are in charge of running the program. These central control stations are responsible for checking the monitoring stations and publishing their measurements on the web.

Two-way communication between the monitoring stations and the central control stations is achieved via the mobile telephony network. The monitoring stations are certified electrical field monitoring devices. They can record the fields emitted across the greater part of the radio spectrum which covers human activity, in other words fields generated by radio and television station transmitting antennas, mobile telephony antennas, etc.

The monitoring stations "quantify" the electromagnetic radiation, measuring electrical field strength (in volts per meter - V/m) and record the effective value every 6 minutes. In other words they record 10 measurements an hour or 240 measurements a day. At the end of the day these measurements are sent to the relevant central control station. Following, the measurements are published on the web in the form of power flux density diagrams (Watts per square meter - W/m²) versus time. Power flux density is used in publishing results instead of electrical field strength because at wave level conditions -in other words when the distance from the antenna is considerably larger than the antenna dimensions- these two figures are interrelated via a simple mathematical formula, and if one is known, it can be used to estimate the other.

Communication between the monitoring stations and the PC at the central control station is achieved via a special GSM modem. It is known that normal modems allow communication between PCs across a fixed telephony network in order to permit connection to the internet. A GSM modem allows communication between PCs over a mobile telephony network. Given that the monitoring stations are equipped with such modems, the administrator can connect to monitoring stations at any given time (a process similar to placing a call on a mobile phone) in order to transfer the stored measurements to the central control station PC.

Once the measurements are received from the monitoring stations they are stored in a database. This database contains a complete dataset for each measurement, in other words, the station of origin, the date and time it was recorded, the 6-minute average effective value, the maximum recorded effective value in the specific 6-minute period, monitoring station temperature and so on. The 6-minute time period has been adopted in Greece as in other countries as the measurement and benchmarking period at reference levels (safety levels). When visitors of the website request information, a database search is performed and the data generated are presented in diagram format to make it easier for the public to comprehend.

Visitors of the HERMES program website can "generate" power flux density graphs for the overall frequency range measured (100KHz to 3GHz) by selecting the monitoring station and time period they are interested in. (Diagram 2).

Diagram 2 - Graph showing average power flux density values for the total frequency range

Moreover, it is possible to generate graphs of power flux density for the frequency bands that mobile telephony antennas operate at (GSM-900 and GSM1800 & UMTS). (Diagram 3).

Diagram 3 - Graph showing average power flux density values for the GSM-1800 & UMTS frequency range

In both these diagrams a comparison is made with the stricter (lower) reference level in each frequency band (the dotted lines) set by Greek legislation (Joint Ministerial Decision No. 53571/3839 on measures to protect the public from the operation of land-based antennas, Government Gazette 1105/B/6.9.2000, and Law 3431/2006 on electronic communications and other provisions, Government Gazette 13/A/3.2.2006). For the overall frequency band, the stricter (lower) reference level has a value of 1.2 W/m² (the frequency band that FM radio broadcasts operate at) while for the two operating frequency bands of the mobile telephony antennas (GSM-900 and GSM-1800 & UMTS) the stricter reference levels have values of 2.7 W/m² and 5.4 W/m² respectively, figures which have been set by the Greek Atomic Energy Commission (GAEC Fact sheet: Mobile Telephony and Health - Questions & Answers).

Diagram 4 - Spectrum Analyzer

The narrow-band measurements system uses spectrum analyzers to carry out the measurements. Spectrum analyzers (Diagram 4) provide on-screen information about spectral content (frequency distribution) and present diagrams of power flux density versus frequency. Radiation receivers, such as antennas (Diagram 5) are also required.

Diagram 5 - Biconical antenna

If an antenna is located in an electromagnetic field with a suitable frequency, then a voltage is generated at its edges, which is directed via a coaxial cable to the input of the spectrum analyzer. The link cable should have low signal loss over the entire range of the frequency spectrum measured and very good shielding to avoid parasitic interference, in other words to ensure that the signal which reaches the spectrum analyzer input is only the signal received from the antenna. The recorded quantity is the power flux density which is expressed in watts per square meter (W/m²). The results of these measurements are then processed, analyzed and presented on this website in bar chart format.

Visitors of this website can see just how much below the strictest limit each spectrum sub-band is in terms of the power flux density value in each spectrum sub-band examined (Diagram 6).

Diagram 6 - Power flux density for each spectrum band

Moreover, they can also view the contribution of each spectrum sub-band to the overall electromagnetic burden for thermal impact conditions (see Joint Ministerial Decision No. 53571/3839/6-9-2000).

Diagram 7 - Percentage contribution of each sub-band